A key limiting factor in the widespread adoption of proton exchange membrane fuel cell (PEN/WC) based power systems is hydrogen fuel storage. The development of a viable hydrogen storage solution will have a profound impact on how consumers will power portable devices, since batteries simply cannot match demands for runtime, energy density and reliability.
Because hydrogen has poor energy content per volume (0.01 MJ/L at standard temperature and pressure and 8.4 MJ/L for liquid hydrogen vs. 32 MJ/L for petroleum), physical transport and storage as a gas or liquid is impractical. Additionally, the compression process to achieve the pressures necessary to reach a high density is energy-intensive and doesn't solve the hazard issue. Also, the densities of compressed H2 or liquefied H2 are still below those required to reach practical fuel storage goals.
Physical means to store hydrogen include sorbents such as carbon nanotubes and foams, zeolites, metal-organic frameworks; and intermetallics such as titanium-manganese alloy 5800, complex hydrides such as metal alanates, amides, and borohydrides, and chemical hydrides such as sodium borohydride/water and ammonia borane (AB). Despite intensive and elegant work on sorbents and complex hydrides, practical systems that can store and release >6 wt % hydrogen at moderate temperatures are still far from realization.
Aluminum hydride (alane) is an attractive candidate for solid hydrogen storage. Alane's formula is sometimes represented with the formula (AlH3)n because it is a polymeric network solid. Alane is formed as numerous polymorphs: the alpha (a), alpha prime (a′), beta ((3), delta (8), epsilon (c), zeta (0, or gamma (y) polymorphs. Each of the polymorphs has different physical properties and varying stability. The most thermally stable polymorph is a-alane, featuring aluminum atoms surrounded by six hydrogen atoms that bridge to six other aluminum atoms. However, alane, including a-alane, is very reactive with water, including moisture in ambient air (e.g., see “Right to Know, Hazardous Substance Fact Sheet” for Aluminum Hydride, available from New Jersey Department of Health & Senior Services, Right to Know Program, PO Box 368, Trenton, N.J. 08625-0368; http://www.nj.gov/health/eoh/rtkweb). For example, It is known that aluminum hydride reacts violently with oxidizing agents and is not compatible with strong acids and metal salts), unless it has been passivated to make it more stable (i.e., stabilized). Consequently, stabilized alane is generally preferred as a hydrogen containing material from which hydrogen gas is generated to prevent loss of a significant portion of the hydrogen from the alane due to wasteful reactions during manufacturing, shipping and storage. The use of stabilized alane can also minimize the need for special packaging and manufacturing under special conditions.
Alane can react to produce aluminum metal and hydrogen gas when heated, according to the following reaction:AlH3→Al+1.5H2  (reaction 1)
Alternatively, alane can also react with water according to the reaction:AlH3+3H2O→Al(OH)3+3H2  (reaction 2)
However, the hydrolysis of stabilized alane is not normally effective for producing hydrogen gas at a reasonable rate for most applications.
Alane can be prepared by several different processes, such as those disclosed in U.S. Pat. Nos. 3,852,043; 6,228,338; and 6,617,064. Several different processes for stabilizing or reducing the reactivity of alane are disclosed in the prior art, such as: (a) storing in an inert atmosphere below 0° C. for an extended period of time (U.S. Pat. No. 3,852,043); (b) contacting with an aqueous buffer at pH 7 at 70° C. (U.S. Pat. No. 3,821,044); (c) treating with a liquid including an organic compound and a small amount of water (U.S. Pat. No. 3,869,544); and (d) washing with 10 w/w percent HCl (U.S. Pat. No. 6,228,338 and U.S. Pat. No. 6,617,064).
In view of the above, an object of the invention is to provide a method of producing hydrogen gas at a relatively high rate by the hydrolysis of stabilized alane.
Another object of the invention is to provide a hydrogen generator capable of producing hydrogen gas at a relatively high rate by the hydrolysis of stabilized alane.